Preheat oven to 200˚C. Rub chicken all over with salt, pepper and olive oil. Place in a roasting dish and roast for ± 60minutes until cooked through. Leave to rest for 20 minutes.
Shred chicken and discard the skin. Use carcass for creating wonderful homemade chicken stock. Heat butter and olive oil in a Dutch oven or deep sauté pan. Add the onion, leek and celery - cook until tender. Add the garlic, dried herbs and fresh thyme.Cook until the garlic is fragrant. Add the flour and cook for a minute, coating the vegetables well. Pour in the chicken stock, stirring quickly as you go to incorporate the flour into the liquid. Add the milk and bring to a simmer. Simmer for about 5 minutes until thickened. Stir in the peas and the shredded chicken. Taste to adjust seasoning. Remove from the heat and set aside.
Heat a drizzle of olive oil in a large frying pan. Sauté mushrooms, in batches if necessary, until golden brown and they have released all of their liquid. Season lightly. Fold golden brown mushrooms into the rest of the filling.
Preheat oven to 180˚C.Select four large ramekins, baking dishes or little cast iron mini pots or mini pans. On a lightly floured surface, roll out the puff pastry slightly and cut out pie lids using the ramekins to measure.Fill the dishes with the chicken and mushrooms filling. Top with the puff pastry, press the edges down to seal against the ramekins. Cut four little slits into each pie crust. Brush all over with egg wash. Bake for ± 35 minutes until deeply golden and the puff pastry is cooked through and perfectly crispy. Serve pies with a light side salad and enjoy!